What’s meant by an “opened email” in the statistics?

If an email is counted as opened in statistics, the recipient has either clicked a link in the message or the pictures have been loaded in their email program. 

A Quick Update for Lime Marketing Customers: In late 2020, Apple launched its iOS 15 update with new privacy controls, including Mail Privacy Protection. As a Lime Marketing customer, this feature enables your recipients who use Apple Mail as their email client to conceal whether or not they've opened an email. The feature works by stopping “senders from using invisible pixels to collect information about the user” (Source). As a result, some of the email statistics you track in Lime Marketing may be impacted by Apple's new privacy features.